This invention concerns a motion detector for the monitoring or control of objects with at least one sensor for the reception of electromagnetic radiation, in particular in the infrared range, from a radiation receiver having a field of sensitivity surrounding the object in an at least partially closed manner, together with an evaluating circuit to trigger a control signal in case of a change in radiation intensity.
Devices of this type, known in practice and having the configuration of passive infrared motion detectors, are used for example as sensing units in door openers, wherein the field of sensitivity of a single sensor of the motion detector located above the door extends to cover an angular range of 180.degree. around the door to be opened. The closing of the door is effected when, within a certain period of time, there is no further change in the radiation intensity within the aforementioned field of sensitivity. In this configuration, the field of sensitivity is produced by means of an expensive multimirror optical assembly, which renders the device about as costly as the radar devices also known for this purpose. Furthermore, no completely closed field of sensitivity may be produced by these devices, which have a maximum angular range of up to 180.degree., so that the known motion detector is unsuited to numerous practical applications.